PWM (pulse width modulator) drivers are often used to drive electromagnetic actuators or devices with coils. The PWM driver is attractive because it can effectively drive heavy and inductive loads with little power loss in the driver throughout the entire control range (0–100% duty cycle).
When operating, the coil acts on some mechanical object by means of a magnetic field created by a current in the coil. The magnitude of the magnetic field is directly proportional to the current in the coil so it is important to control or monitor this current. The magnitude of the current can be predicted by dividing the average voltage across the coil by an assumed coil resistance. Unfortunately, because the coil resistance is a strong function of temperature and temperature can change dramatically as the coil is being driven, this prediction is often insufficient.
Several methods of measuring the current being driven into a coil from a PWM driver have been used. Most of these methods suffer by being inaccurate or costly. For example, one typical approach is to take a PWM driver circuit, typically a half bridge circuit built with MOSFETs (metal oxide semi conductor field effect transistors) or other transistors, and placing a current measuring circuit after the PWM driver circuit in series to measure the output of the PWM driver circuit. Because the measuring circuit is in series with the output of the PWM driver circuit and because the PWM driver circuit is a PWM driver, the measuring circuit must measure the current accurately as the voltage on the output signal is constantly being switched from ground to source voltage. The measuring circuit must also transmit this information back to a receiving device. The receiving device is typically a micro controller and typically referenced to ground, so the measuring circuit needs to reject the wide common mode voltage swing and measure only the current in the output signal and feed this back to a monitoring or controlling device referenced to the ground of the receiving device. Because of all these variables, measuring circuits are typically extremely costly to design and produce inaccurate results.
Therefore, there is a need in the art to provide for a current measuring circuit for a PWM driver that is both inexpensive to design and accurate in its measurements. There is a further need to minimize the effects to a current measuring system attempting to measure an output signal that is constantly being switched from ground to source as in a PWM driver.
Thus, it is a primary object of the present invention to provide a current measuring circuit for a PWM driver that improves upon the state of the art.
Another object of the present invention is to provide a method of measuring the current of a PWM driver using a measuring circuit that will not be affected by the constant switching of ground to source voltage of the PWM driver.
Yet another object of the present invention is to provide a current measuring circuit for a PWM driver that will be inexpensive to create and produce an accurate measurement.
Another object of the present invention is to measure the current supplied to a PWM driver that drives a coil of an electrohydraulic valve.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.